Tuning means



1946- H: c. LAWRENCE, JR 2,

TUNING MEANS Filed Feb. 17', 1944 inventor Patented Oct. 29, 1946 UNITED STATES PATENT OFFICE TUNING MEANS Howard C. Lawrence, Jr., Haddonfield, N. J., asl signor to Radio Corporation of America, a corporation of Delaware Application February 17, 1944, Serial No. 522,726

9 Claims. (Cl. 250-40) l v 2 The invention covered herein may be manufaccauses sparking which burns the contacts and tured and used by or for the Government of the thus increases the rate at which failure may United States for any governmental purpose withoccur. out payment to me or assigns of any royalty Accordingly, the principal object of the present thereon. 5 invention is to obviate the foregoing and other This invention relates to improvements in tunless apparent objections to present day high freing apparatus and particularly to improvements quency tuners. Y in variable tuners for use in radio, radar and Another and related object of the invention is analogous high and ultra-high frequency systems to provide an improved continuously variable for the communication of intelligence. tuner suitable for use at high and ultra-high fre- It is well known to those skilled in the art to quencies and one characterized by the simplicity which this invention appertains that the problem and economy of its parts and by its absence of of tuning high and ultra-high frequency circuits slidable contacts or other adjustable elements of is far more complex than that of tuning relatively a, type presenting a substantial impedance to the low frequency circuits. This is so because the flow of current at the frequencies indicated. circuit parameters and the circuit components Other objects and advantages together with become increasingly small with an increase in certain referred details of construction will be frequency. Thus, the variable capacitor method apparent and the invention itself will be best of tuning radio circuits becomes practically ununderstood by reference to the following specifi workable at very high frequencies, especially in cation and to the accompanying drawing wherein: high powered transmitters because the spacing Figs. 1 and 2 are views in perspective of a radio necessary to prevent voltage breakdown makes it transmitter installation embodying an improved impossible to provide sufficient capacity to tune tuner within the present invention, and the circuit with elements which are small com- Fig. 3 is a similar view of an alternative em- Pared w a e th. Convent onal variable bodiment of a tuner within the invention and inductance methods become unworkable at very wherein the tuner is designed to handle a relahigh frequencies for a like reason and also betively wide wave band of a lower order of frecause there is often insumcient space available quencies than the apparatus of Figs. 1 and 2. to couple the inductor to the line. In order to In the accompanying drawing wherein like refobviate the foregoing difliculties it has previously erence characters designate the same or correbeen the practice to employ a transmission line spending parts in all figures, I designates a chassis as the tunable circuit in high frequency installaor panel of a radio apparatus and 3 and 5 desigt on a d to im t e ne by Var i s effective nate, respectively, two vacuum tubes which are length, as by means of a movable shorting bar mounted in spaced relation upon the said panel. or plug, or by means of a variable capacitor These tubes have dependent grid leads I, la reconnected across the line or a small inductor spectively, which extend below the underside of variably coupled to one end of the line. The the panel I and the said leads are connected to movable shorting bar method has been, up to the opposite ends of a fiat strip 9 of thin connow, the only practical tuning method available ducting material such, for example, as phosphor for many uses. Shorting bars, however, are not 46 bronze. The strip 9 is of a length substantially always satisfactory, principally because their use greater than the space between the electrode involves slidable or other friction type contacts. elements or leads I, la, to which it is connected, These contacts are very often at a point of high and may be said to comprise an inductive loop current and therefore the resistance must be of less than one full turn. A rack II is connected especially low to prevent a reduction in the eflito the midpoint of the strip or lead 9 through an ciency of the circuit. This is not always practical insulating element or grommet l3 and is driven because the contacts may become dirty in use by a pinion H). The rack H and pinion l5 proand prevent the maintenance of a low resistance vide a convenient mechanism for altering the path for the current. Further, dirty contacts area embraced by the loop and hence the efiecresult in the introduction of noise into a receiver 50 tive inductance of the strip 9 comprising the said circuit and, their presence in a transmitting cirloop. A flat rigid strip ll constituted of insulatcuit results in a reduction in usable power. It is ing material prevents the loop from buckling also had practice to tune a transmitter by means when it is compressed. of a movable shorting bar when the power is on The inductance L provided by this flat conducbecause the movement of the sh rting ar u lly tive strip or loop 9, together with the relatively fixed interelectrode capacitance C within the tubes 3 and 5, comprise a tunable L-C circuit wherein tuning is achieved by varying the area embraced by, and hence the effective inductance of, the loop.

In Fig. 1 the loop 9 is shown in its expanded (maximum area, maximum inductance) position, and Fig. 2 shows the loop in its compressed (minimum area, minimum inductance) position.

It should be noted that since the conductor 9 comprises a flat strip, it can flatten out in one direction only, hence it will not twist or buckle as it would if a round or square cross-section conductor were used. Further, the flat strip here illustrated exhibits a low radio-frequency resistance because the current flows mostly on the side surface of the loop and not on the thin edges.

As previously pointed out, the driving mechanism II, I is insulated from the center of the conductive strip or loop 9 by the insulator [3. This permits a direct current connection 19 to be made at this point to the grid bias supply (not shown).

As the loop 9 is compressed or expanded by the driving mechanism ll, l5, the effective area embraced by the loop is altered without relatively moving the ends of the loop 9 which, as above pointed out, are rigidly secured to the grid leads I, la. This change in the area embraced by the loop results in a change in inductance, the inductance being less when the loop is compressed than when it is expanded. The compressed loop therefore tunes the circuit to the higher frequencies. The inwardly extending oppositely located bends in the flexible metal strip 9 will be folded quite sharply when the loop is entirely compressed and when thus folded act as by-pass capacitors in the extreme compressed position and therefore short out part of the length of the strip, thereby further increasing the available tuning range.

The tuning system shown in Figs. 1 and 2 has been used with success in a high powered (100 kilowatt) radar transmitter. In this case the tubes 3 and 5 comprised a pair of Navy type CV92 (RCA type 8026) tubes. The tuning loop 9 comprised a phosphor bronze strip V inch wide and 0.010 inch thick and about 7 inches long. The tubes 3 and 5 were spaced with their centers 3%; inches apart. The movement at the center of the loop was A of an inch. This gave a frequency range of at least 205 to 260 megacycles. The insulating backing strip IT was 4 inches long and of an inch wide. In comparative tests with a conventional variabie capacitor tuning system, the prior art system exhibited a tuning range of I only megacycles without sparking-over and another prior art tuning system employing a variable coupling to a small loop exhibited a similar inadequate coverage. It should be mentioned that the use of the insulating strip I! to back up the conductive strip 9 prevents the loop from assuming a heart-shape configuration when the adjusting force is applied at its mid-point and this contributed materially to the successful operation of the apparatus over the above-mentioned tuning range. (When the backing strip was omitted the tuning range was very much smaller, say 200-220 megacycles.)

Referring now to the embodiment of the invention shown in Fi 3: A still wider tuning range is achieved by providing the flexible loop 9 with an accordion pleat or fold 9a intermediate its ends. As this accordion fold 9a is compressed or expanded by the force applied to the strip 9 by the driving mechanism I3, I5, the area embraced by the said strip or loop is more greatly altered and the resulting tuning range is several times that exhibited by the simple loop of Figs. 1 and 2. A fixed capacitor, which is indicated symbolically at H, may be connected across the ends of the loop 9 to augment the interelectrode capacitance of the tubes 3 and 5 and thereby render the tuning system of the invention operable at frequencies lower than those mentioned above.

Other modifications and applications of the invention will suggest themselve to those skilled in the art. Accordingly, the foregoing description of certain preferred embodiments of the invention should be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

1. In combination, an electrically resonant circuit wherein the capacitance is fixed and the inductance comprises a flexible conductive loop of less than one full turn, and means for deforming said loop to vary the area embraced by and hence the effective inductance of said loop.

2. An electrically resonant circuit comprising a flexible conductive loop of less than one full turn, means for securing the ends of said loop against relative movement, means establishing a substantially fixed value of capacitance between the said ends of said loop, and mean for flexing said loop to vary the area embraced by and hence the effective inductance of said loop, whereby to vary the electrical resonance of said circuit.

3. Radio tuning apparatus comprising a sup port, a pair of circuit elements mounted spaced relation upon said support, said circuit elements each possessing a substantially fixed value of capacitance, a flexible conductor connected adjacent to one of its ends to one of said circuit elements and adjacent to the other of its ends to the other of said circuit elements, said conductor being longer than the distance between said spaced circuit elements and comprising an inductive loop of less than one full turn, and means connected to said flexible conductor intermediate its ends for altering the contour of said loop, the effective value of its said inductance, and the frequency to which said radio apparatus is tuned.

4. The invention as set forth in claim 3 and wherein said circuit elements comprise a pair of vacuum tubes each containing a control grid, and said flexible conductor is connected at each of its opposite ends to one of said control grids.

5. The invention as set forth in claim 3 and wherein a capacitor is connected across the open ends of said loop to augment the said fixed value of capacitance.

6. In combination, a pair of capacitive circuitelements, a flexible conductive strip connected at its opposite ends to said pair of capacitive circult-elements, said circuit-elements being spaced apart a distance less than the length of said strip to form an inductive loop of less than one full r turn, and adjustable means connected adjacent to the mid-point of said strip for altering the area embraced by said loop and hence the effective inductance of said conductive strip.

'7. The invention as set forth in claim 6 and wherein said conductive strip is provided with an accordion fold intermediate its ends.

8. The invention as set forth in claim 6 and wherein a rigid piece of insulating material is provided intermediate said adjusting mean and said conductive strip for controlling the contour of said conductive Strip when the area embraced by said loop is altered by said adjusting means.

9. An inductively tuned circuit comprising a resilient conductor of predetermined length and constituting substantially the entire inductance of said circuit, means for securing the ends of said conductor against relative movement, means for establishing a substantially fixed value of capacitance between said ends of said conductor, said last mentioned means constituting substantially the entire capacitance of said circuit, and means for flexing said conductor whereby to vary its eifective inductance and hence the tuning of said circuit.

HOWARD C. LAWRENCE, J R. 

